The present invention, in some embodiments thereof, relates to compositions and methods for inducing thrombopoiesis.
Platelets are circulating cell-derived fragments that are required for the maintenance of hemostasis. These small, anucleate fragments represent the first line of defense against hemorrhage following vascular injury, and are crucial for blood coagulation. Thrombopoiesis is the complex process of platelet production from megakaryocytes (MKs), and even to date is incompletely understood. Functional platelets are the terminal differentiation product of this process. Specifically, in the bone marrow (BM), MKs give rise to circulating platelets through commitment of the multipotent hematopoietic stem cell to the MK lineage, proliferation of the progenitors and terminal differentiation of MKs by blebbing of their membrane and production of platelet fragments. This process is characterized by DNA endoreplication, followed by cytoplasmic maturation and expansion. Platelets form as the fully mature MK develops cytoplasmic extensions, or pseudopodial protrusions, that extend in proximity to sinusoidal endothelial cells (Deutsch V R and Tomer A. Br J Haematol. 161(6):778-93 (2013), Tvassoli and Aoki, Blood Cells, 15:3-14 (1989)). Platelets bud from the ends of these protrusions and thereafter enter the circulation. The ability of MK to produce platelet buds is ultimately exhausted, and it undergoes terminal apoptosis.
A number of diseases, such as ITP and TTP, or conditions result in low levels or poor functioning of blood platelets. For example, pancytopenia and prolonged thrombocytopenia, which remain a significant clinical challenge for patients undergoing hematopoietic stem cell transplantation and high dose chemotherapy. Chemotherapy or irradiation-associated depletion of hematopoietic precursors in the BM results in hemorrhagic and life threatening infectious complications. Engraftment of transplanted cells or regeneration of normal hematopoiesis and blood count recovery is usually accomplished within 2 to 5 weeks. The reason for this delay has been attributed to insufficient MK precursors in the grafts48.
Currently available treatments for thrombocytopenia and related conditions include, for example, corticosteroids, IVIG, splenectomy, and whole blood or platelet transfusion, methods which are either palliative and non-specific, or drastic and expensive. Platelets for such procedures are obtained by plateletphoresis from normal donors. However the efficiency of such costly transfusions can be limited since platelets have a relatively short shelf-life of about 5 days. Furthermore, patients are often refractory to subsequent transfusions. Injections of Thrombopoietin (TPO), the physiologic regulator of thrombopoiesis, and TPO mimetics to increase platelet count has not been clinically effective due to a lag period before the level of platelets was affected.
Thus, there remains a need for new and improved methods for stimulating or enhancing the production of platelets.
The IAP family of proteins has been shown to inhibit apoptosis induced by a variety of stimuli mainly by binding and inhibiting specific caspases25. Eight human IAPs have been identified to date: c-IAP1, c-IAP2, NAIP, Survivin, XIAP, Bruce, ILP-2 and Livin.
Livin, also known as baculoviral IAP repeat-containing 7; BIRC7, MAP, ML-IAP and Livin inhibitor-of-apoptosis, contains a single baculovirus IAP repeats (BIR) domain at the N-terminus and a carboxy-terminal RING domain29-31. The BIR domain was shown to play a role in the anti-apoptotic function of IAPs26,27. Livin encodes two highly similar splicing variants, termed Livin α and 13 that differ only in 18 amino acids located between the BIR and the RING domains, which are present in the α but not in the β isoform. Following apoptotic stimuli, both Livin isoforms α and β undergo a specific proteolytic cleavage that trims the 52 amino acids at the N-terminus of Livin. From each isoform a truncated C-terminal Livin is thus produced, of approximately 30 kDa (also termed p30) and 28 kDa (also termed p28), respectively, containing the full BIR and RING domains (Ashhab, Y. et al. FEBS letters, 495: 56-60 (2001)). These truncated forms of Livin are collectively referred to as tLivin. Recent reports show that tLivin is not only devoid of Livin anti-apoptotic activity but also acquires a pro-apoptotic effect32,33. Thus, Livin is unique among the IAP members, exerting both anti-apoptotic and pro-apoptotic activities making it a regulator of apoptosis rather than anti-apoptotic protein32,33.
U.S. Pat. No. 7,517,949 discloses Livin-derived peptides with pro-apoptotic activity. Specifically provided are peptides p30-Livin α and p28-Livin β, derived from Livin α and β truncation, respectively, as well as compositions thereof. These peptides display pro-apoptotic activity and as such are used for the enhancement and/or induction of apoptosis, as well as for the treatment of cancer.